I heard of a company called planet numbers and another called cherry calls that have very cheap prices but honestly I haven't tried any and I am not sure which is best. I guess you could find the information you are looking for on the net. Do you already have mobile that you are using in the UK? Most providers enable you to call abroad, you'd have to ask what the costs are.

 

I use lycamobile, just a sim card, i just top up when i need, no subscribtion, the fares are very interesting, have a look on their website www.lycamobile.co.uk